1. Field of the Invention
The present invention relates to the an optical communication system which combines a plurality of signal lights into a wavelength division multiplexed signal light for transmission through an optical fiber transmission line. More particularly, the present invention relates to an arrangement of frequencies of the signal lights to eliminate the effects of four-wave mixing (FWM).
2. Description of the Related Art
Optical communication systems can provide long-distance, large capacity transmission. Therefore, such optical communication systems will likely be used for future multimedia networks which require these characteristics.
Various techniques are being studied for increasing the capacity of optical communication systems. Such techniques include the use of time-division multiplexing (TDM), optical time-division multiplexing (OTDM) and wavelength-division multiplexing (WDM). Of these techniques, WDM is considered the most advantageous, since the transmission speed for each optical carrier signal light can be set at a lower value for the same transmission capacity, thereby resulting in greater tolerance to wavelength dispersion and nonlinear optical effects of an optical fiber transmission line.
Moreover, the development of erbium-doped fiber amplifiers (EDFA) having a wide xe2x80x9cgain regionxe2x80x9d (that is, a wavelength region where a desired gain is obtained) has allowed optical communication systems to sufficiently amplify wavelength division multiplexed signals, thereby increasing the use of WDM. It is hopeful that WDM will allow optical communication systems to provide a flexible lightwave network in which cross connects, branching, insertion, and multiplexing of different kinds of services can be performed at optical levels by utilizing the wide gain region of an EDFA.
However, waveform degradation caused by wavelength dispersion of an optical fiber becomes significant in optical communication systems where the transmission speed for each optical carrier signal light exceeds several gigabits per second. Moreover, with long-distance transmission, input power to an optical fiber transmission line must be increased to obtain a required signal-to-noise ratio (SNR) . Unfortunately, this increase causes an increase of self-phase modulation (SPM), a nonlinear optical effect in optical fibers.
SPM causes many problems. For example, wavelength chirping of signal light by SPM causes waveform degradation through interaction with the group velocity dispersion (GVD) of an optical fiber, thereby providing an SPM-GVD effect. To eliminate or suppress the SPM-GVD effect, a dispersion-shifted fiber (DSF) at 1.55, xcexcm can be used as an optical fiber transmission line to minimize the dispersion value at signal light wavelength. However, in WDM transmission using the low-dispersion region of the DSF, the occurrence of a nonlinear optical effect in optical fiber, known as four-wave mixing (FWM) between signal lights, will become noticeable. The effect of FWM manifests itself in the form of crosstalk and attenuation. More specifically, crosstalk is caused by the selection and reception of FWM light together with the signal wavelength. Attenuation of the signal light is caused by energy transfer to the FWM light. Crosstalk and attenuation degrade SNR, and, in the worst case, make transmission impossible.
The occurrence efficiency of FWM increases as the transmission channel dispersion decreases, signal light power increases, wavelength separation reduces, or the number of wavelengths increases. Since FWM occurs at a lower power level than other nonlinear optical effects, the effects of FWM tend to easily occur. In reality, the effects of FWM increase because signal light power must be increased by using a low-dispersion region of a DSF transmission line and transmissions must be performed using closer channel spacing due to the limited signal band resulting from the wavelength-dependence of optical components and the gain region of optical amplifiers. Accordingly, sufficient consideration must be given to the effects of FWM in designing a WDM system.
Various methods have been proposed to suppress FWM. Such methods include separating the signal band substantially away from the zero dispersion wavelength xcexo of an optical fiber transmission line (H. Miyata et al., xe2x80x9cStudy on the Effects of FWM in WDM Transmission Considering Fiber Dispersion Variationsxe2x80x9d, Technical Report, The Institute of Electronics, Information and Communication Engineers, SSE93-143, OCS93-73 (1994-03); N. S. Bergano et al., xe2x80x9c100 Gb/s WDM Transmission of Twenty 5 Gb/s NRZ Data Channels Over Trcosoceanic Distances Using a Gain Flattened Amplifier Chainxe2x80x9d, Proc. 21st Enr. Conf. on Opt. Comm. (ECOC ""95xe2x80x94Brussels)). However, with this method, the signal band may undesireably shift from the gain region of optical amplifiers. Also, the zero dispersion wavelength xcexo must be managed with high accuracy. Further, dispersion compensation becomes necessary for long-distance transmission because fiber dispersion increases.
Accordingly, it is an object of the present invention to provide a WDM optical communication system that can substantially eliminate or suppress the effects of FWM.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and, in part, will be obvious from the description, or may be learned by practice of the invention.
Objects of the present invention are achieved by providing an optical transmitting device which combines a plurality of signal lights, each having a different, corresponding, frequency, into a wavelength division multiplexed signal light. For at least three signal lights of the plurality of signal lights, the difference in frequencies of any pair-combination of the at least three signal lights is different from the difference in frequencies between any other pair-combination of the at least three signal lights.
Moreover, objects of the present invention are achieved by providing the at least three signal lights to include n signal lights having respectively corresponding frequencies f1 through fn arranged in order from f1 to fn along a frequency spectrum, where frequencies f1 through fnxe2x88x921 have respectively corresponding integer spacing coefficients m1 through mnxe2x88x921, and frequencies fi and fi+1 are separated by mixc2x7xcex94fSxe2x88x92F, where i=1 to (nxe2x88x921) and xcex94fSxe2x88x92F is a unit of spacing between frequencies. m1 through mnxe2x88x921 should all be different from each other. In addition, m1 through mnxe2x88x921 and the sum of any consecutive (nxe2x88x922) spacing coefficients m1 through mnxe2x88x921 should all be different from each other. m1 through mnxe2x88x921 should be selected to minimize the sum of m1 through mnxe2x88x921, and thereby reduce the required bandwidth.
Objects of the present invention are also achieved by providing an optical transmitting device for combining first, second and third signal lights, each having a different, corresponding, frequency, into a wavelength division multiplexed signal light. The difference in frequencies of any pair-combination of the first, second and third signal lights is different from the difference in frequencies between any other pair-combination of the first, second and third signal lights.
Moreover, objects of the present invention are achieved by providing the first, second and third signal lights so that the first, second and third signal lights have frequencies f1, f2 and f3, respectively, arranged in order from f1 to f3 along a frequency spectrum, where frequencies f1 and f2 have respectively corresponding integer spacing coefficients m1 and m2, frequency f2 is separated by m1xc2x7xcex94fSxe2x88x92F from frequency f1, and frequency f3 is separated by m2xc2x7xcex94fSxe2x88x92F from frequency f2. xcex94fSxe2x88x92F is a unit of spacing between frequencies. m1 and m2 are different from each other.
In addition, objects of the present invention are achieved by providing an optical transmitting device for combining a plurality of signal lights, each having a different, corresponding, frequency, into a wavelength division multiplexed signal light. The frequencies of a first arbitrary pair of signal lights of the plurality of signal lights have a frequency difference defined as a first frequency difference. The frequencies of a second arbitrary pair of signal lights of the plurality of signal lights, and different from the first arbitrary pair, have a frequency difference defined as a second frequency difference. The absolute value of the first frequency difference is different from the absolute value of second frequency difference.
Objects of the present invention are further achieved by providing an optical communication system which includes transmitters, a multiplexing device, an optical fiber transmission line, a demultiplexing device and a receiver. The transmitters produce signal lights, each signal light having a different, corresponding, frequency. The multiplexing device combines the signal lights into a wavelength division multiplexed signal light. The optical fiber transmission line receives the wavelength division multiplexed signal light from the multiplexing device and propagates the wavelength division multiplexed signal therethrough. The demultiplexing device receives the wavelength division multiplexed signal light from the optical fiber transmission line and demultiplexes the received wavelength division multiplexed signal light into individual signal lights. The receiver receives at least one of the individual signal lights from the demultiplexing device. For at least three signal lights of the plurality of signal lights produced by the transmitters, the difference in frequencies of any pair-combination of the at least three signal lights is different from the difference in frequencies between any other pair-combination of the at least three signal lights.
Objects of the present invention are also achieved by providing a method which includes the steps of: (a) providing a plurality of signal lights, each having a different, corresponding, frequency, wherein, for at least three signal lights of the plurality of signal lights, the difference in frequencies of any pair-combination of the at least three signal lights is different from the difference in frequencies between any other pair-combination of the at least three signal lights, and (b) combining the at least three signal lights into a wavelength division multiplexed signal light.